1. Field of the Invention
The present invention relates to a method and an apparatus for aiding image interpretation and to a computer-readable recording medium storing a program therefor. More specifically, the present invention relates to a method and an apparatus for aiding comparative reading of two images that are not adjacent to each other among three or more images arranged in temporal order for representing a change in the state of a subject, and to a computer-readable recording medium storing a program therefor.
2. Description of the Related Art
Comparison and observation of only type of change, based on a plurality of images obtained by photography of a subject showing simultaneous changes of two or more types, is necessary in some cases. In this case, by removing an effect of the change or changes of the other type or types that are not a target of observation, the change of the type as the observation target can efficiently be compared and observed with high accuracy.
For example, diagnosis of respiratory function is attempted based on simple X-ray images representing different phases of the respiratory cycle. In this case, the X-ray images include two types of changes, namely movements of anatomical structures such as ribs and the diaphragm and a change in density in lung fields. Among those changes, the change in density in lung fields is effective for diagnosing pulmonary emphysema or the like accompanying airflow obstruction (an obstructive respiratory disorder) as one type of chronic obstructive pulmonary diseases.
In the case of pulmonary emphysema, an increase in transmissivity in lung fields is shown in a chest X-ray image, reflecting destruction or loss of alveoli and a decrease in lung vessels caused by emphysema, and reflecting abundance of air caused by over-inflation of lungs. In addition, pathological pulmonary emphysema is reflected in a low attenuation area in a chest CT image (Kawamoto and Kanbe, “7. Chronic Obstructive Pulmonary Diseases (in Japanese)”, Fourth Plan for Guideline 2002 for clinical examination corresponding to DRG/PPS, Japanese Society of Laboratory Medicine, November 2002, p26). Therefore, in the case of observation of density change in lung fields based on X-ray images representing different respiratory states, an area wherein density and X-ray absorption do not change greatly is detected as an area of abnormal respiration caused by air that remains in lung fields and is not removed by exhalation.
As an attempt to diagnose a respiratory function using simple X-ray images, abnormal respiration can be detected based on a moving image obtained by photography with an image intensifier for representing different phases of respiratory cycle starting from inhalation and ending in exhalation. In order to detect such abnormal respiration with high accuracy, alignment of the images is reported to be necessary (see J. Liang, et al., “Dynamic chest image analysis: model-based ventilation study with pyramid images”, Proceedings of SPIE Medical Imaging 1997: Physiology and Function from Multidimensional Images, SPIE, May 1997, Vol. 3033, p 81-92). Furthermore, discussion has also been made on usage of a digital X-ray imaging apparatus adopting a flat-panel detector for moving images that are getting used practically. By using a moving chest image in a resolution higher than a resolution by an image intensifier, information on respiratory function can be obtained more accurately from a change in lung-field density. In addition, as shown in FIG. 24, an analysis has been attempted by generating a moving subtraction image through alignment of two neighboring images in a moving image and calculation of a difference between the two images (see Tanaka, Sanada, et al., Qantitative Analysis of Respiratory Kinetics in Breathing Chest Radiographs Obtained Using a Dynamic Flat-Panel Detector”, Journal of Japan Society of Medical Imaging and Information Sciences, Japan Society of Medical Imaging and Information Sciences, January 2003, Vol. 20, Issue 1, p 13-19).
Meanwhile, a temporal subtraction technique is known for generating a subtraction image based on a difference between two images through alignment of the same subject in the two images obtained by photography at different times (see Japanese Unexamined Patent Publications No. 7(1995)-037074, U.S. Pat. No. 5,790,690, U.S. Patent Laid-Open Nos. 20010002934 and 20010048757 and A. Kano, K. Doi, H. MacMahon, D. Hassell, M. L. Giger, “Digital image subtraction of temporally sequential chest images for detection of interval change”, Medical Physics, AAPM, Vol.21, Issue 3, March 1994, p. 453-461) This technique is effective in the case where radiographs obtained by photography of the chest of the same subject at present and at one year earlier are compared to each other, for example.
In diagnoses of abnormal respiration of lungs, it is thought to be important to observe a change in density in lung fields through comparison of a maximal inhalation image and a maximal exhalation image.
On the other hand, in the non-patent references written by Liang et al. and Tanaka et al., a serial change is only observed in images representing different respiratory states or in images adjacent to each other in a moving image. Therefore, a density change in lung fields is only indirectly and ineffectively understood between a maximal inhalation image and a maximal exhalation image.
Furthermore, a size change in lung fields is large between a maximal inhalation image and a maximal exhalation image, due to three-dimensional movements of ribs and the diaphragm. In addition, ribs show different movements in different directions from the soft tissue in lung fields. Therefore, direct comparison of a maximal inhalation image and a maximal exhalation image is difficult. For this reason, alignment can be carried out between a subject in a maximal inhalation image and a maximal exhalation image by using alignment processing in the conventional temporal subtraction technique described in Japanese Unexamined Patent Publications No. 7(1995)-037074, U.S. Pat. No. 5,790,690 and U.S. Patent Laid-Open Nos. 20010002934 and 20010048757. However, since the images have too much a difference as has been described above, alignment cannot be carried out with high accuracy.
Consequently, in the case where comparative image reading is carried out between two images that are not adjacent to each other in three or more images arranged in temporal order for representing changes in a subject, observation of a necessary one of the changes alone has been impossible through the comparative image reading even if an effect of the other change or changes is removed after alignment of the subject in the two images.